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Perimesencephalic Cisterns

Last Updated: August 23, 2020

ABSTRACT

OBJECTIVE: To describe the exposure obtained through six approaches to the perimesencephalic cisterns with an emphasis on exposure of the posterior cerebral artery and its branches.

METHODS: Dissections in 12 hemispheres exposed the crural, ambient, and quadrigeminal cisterns and related segments of the posterior cerebral artery. A Stealth Image Guidance workstation (Medtronic Surgical Navigation Technologies, Louisville, CO) was used to compare the approaches.

RESULTS: The transsylvian approach exposed the interpeduncular and crural cisterns. The subtemporal approach exposed the interpeduncular and crural cisterns as well as the lower half of the ambient cistern. Temporal lobe retraction and the position of the vein of Labbé limited exposure of the quadrigeminal cistern. Occipital transtentorial and infratentorial supracerebellar approaches exposed the quadrigeminal and lower two-thirds of the ambient cistern. Transchoroidal approaches exposed the posterior third of the crural cistern, the upper two-thirds of the ambient cistern, and the proximal quadrigeminal cistern. Transchoroidal approaches exposed the posterior portion of the P2 segment (P2p) in 9 of 10 hemispheres and were the only approaches that exposed the lateral posterior choroidal arteries and the plexal segment of the anterior choroidal artery. Occipital transtentorial and infratentorial supracerebellar approaches provided access to the P3 segment in all cases and exposed the P2p segment in 4 of 10 hemispheres. The subtemporal approach provided access to the cisternal and crural segments of the anterior choroidal and medial posterior choroidal arteries and exposed the P2p segment in 3 of 10 hemispheres.

CONCLUSION: Surgical approaches to lesions of the perimesencephalic cisterns must be tailored to the site of the pathological findings. The most challenging area to expose is the upper half of the ambient cistern, particularly the P2p segment of the posterior cerebral artery.

INTRODUCTION

Approaching lesions within the perimesencephalic cistern presents unique challenges to the neurosurgeon. The anatomy of this region is complex, with numerous arteries, perforating vessels, deep venous drainage pathways, and cranial nerves coursing through its confines. The pathological findings of the region are diverse and encompass the entire spectrum of neurosurgery. The perimesencephalic cisterns lie deep within the brain, and portions are shielded by overlying structures, which include the temporal lobe, parahippocampal gyrus, limen insula, tentorial incisura, and vein of Labbé. No single surgical approach can provide access to the entire perimesencephalic cistern, but approaches must be tailored to the site and type of the patient’s pathological findings with a thorough understanding of the anatomy of this region.

The ambient portion of the perimesencephalic cistern and accompanying posterior portion of the P2 segment (P2p) of the posterior cerebral artery (PCA) is a particularly challenging area to expose. Selecting the appropriate surgical approach to this portion of the PCA remains controversial (14, 16, 25, 35, 44, 51). The goals of the present study were to define the anatomic segments of the perimesencephalic cisterns, describe the microanatomy of six approaches to the region, compare the approaches using a novel application of image guidance, and outline a strategy for approaching lesions in this area.

MATERIALS AND METHODS

A total of 12 hemispheres from six cadaveric heads infused with colored silicone were examined in the study. Magnification ×3 to ×40 and microsurgical techniques were used to compare six surgical approaches to the perimesencephalic cisterns. The approaches studied were the transsylvian pretemporal, subtemporal, occipital transtentorial, infratentorial supracerebellar, transtemporal transchoroidal, and transinsular transchoroidal. The segments and major branches of the PCA exposed through each approach were identified in 10 hemispheres. Special attention was paid to the anatomic structures limiting visualization in each approach.

Three cadaveric heads were studied with magnetic resonance imaging and registered on a Stealth Image Guidance workstation (Medtronic Surgical Navigation Technologies, Louisville, CO) using standard protocols. Three-Tesla magnetic resonance imaging scans using a Siemens Magnetom Allegra scanner (Siemens Medical Systems, Inc., Erlangen, Germany) were obtained and transferred to a Stealth workstation. Before scanning, bilateral large orbitozygomatic frontotemporal, bilateral occipital, and suboccipital craniotomies were performed. In essence, three bridges of bone were left intact over the convexity, to which fiducials were attached. The bony bridges were over the superior sagittal sinus and parieto-occipital cortex. Care was taken to maintain an intact dura. Ten fiducial landmarks to be used for image registration were placed in a uniform manner over the bony bridges. The remaining calvaria provided a stable platform for registration and maintenance of relative anatomic positioning during microdissection. A three-dimensional model was built, and a dynamic reference array was attached to the cadaver. The cadaver was registered to the three-dimensional model and image, and an estimated accuracy of better than 2 mm at 10 cm was established. An active probe was then used to indicate the surgical exposure on the three-dimensional model and three orthogonal magnetic resonance imaging views. These images were recorded through screen captures. The limits of exposure through each of the approaches to the perimesencephalic cisterns were determined by image guidance under direct microscopic visualization. Measurements included the superior, inferior, anterior, and posterior limits of surgical exposure as well as the trajectory and working distance for each approach.

Basic Anatomy

The perimesencephalic cisterns consist of cerebrospinal fluid-filled spaces that surround the midbrain. We divide the region into four segments. The first segment is the interpeduncular cistern, which lies between the cerebral peduncles and communicates anteriorly with the suprasellar cistern and superolaterally with the sylvian fissure (3, 24). The second segment is the crural cistern, which lies within a space bounded by the cerebral peduncle medially, the posterior segment of the uncus laterally, and the optic tract superiorly (3, 24). The crural cistern communicates superiorly with the sylvian fissure, medially with the interpeduncular cistern, and posteriorly with the ambient cistern. The third segment is the ambient cistern, which extends from the posterior margin of the crural cistern to the lateral edge of the midbrain colliculi (24). The walls of the ambient cistern are as follows: anteriorly, the posterior surface of the cerebral peduncle; medially, the lateral surface of the midbrain; laterally, the tentorial edge, parahippocampal gyrus, fimbria of the fornix, and choroidal fissure; and superiorly, the pulvinar of the thalamus, lateral geniculate body, and optic tract (Fig. 1). When viewed in a coronal plane, the ambient cistern is shaped like a “C” around the parahippocampal gyrus (Fig. 1D). The final segment is the quadrigeminal cistern, which lies posterior to the colliculi, below the splenium, anterior to the apex of the cerebellum, and superior to the cerebellomesencephalic fissure (Fig. 1, A and D) (3, 22, 24).

Figure 1. A, specimen showing superior view of the perimesencephalic cisterns. The perimesencephalic cisterns include the interpeduncular, crural, ambient, and quadrigeminal cisterns surrounding the midbrain. The interpeduncular cistern lies between the cerebral peduncles. The crural cistern lies between the posterior segment of the uncus and the cerebral peduncle. The ambient cistern extends from the posterior edge of the crural cistern to the lateral edge of the midbrain colliculi. The head, body, and tail of the hippocampus and the choroid plexus are exposed in the floor of the temporal horn. Note how the right P2p courses above the medial edge of the parahippocampal gyrus, whereas the left P2p courses medial to the gyrus. B, specimen showing inferior view of the basal cisterns. The right P2a and P2p have been removed to expose the roof of the crural and ambient cisterns. The lower part of the temporal lobe has been removed to expose the temporal horn and basal cisterns. The anterior choroidal artery enters the choroid plexus in the temporal horn. The lateral geniculate body and optic tract sit in the roof of the cisterns. The P1, P2, and P3 segments; inferior temporal branches; and medial posterior and lateral posterior choroidal arteries are exposed. C, specimen showing anterior inferior temporal lobe below the choroidal fissure removed to expose the relationship between the P2p and choroidal fissure. The choroidal fissure begins at the posterior edge of the posterior uncal segment at the site where the anterior choroidal artery enters the temporal horn to become the plexal portion. Note how the right P2 ascends and courses laterally to reach the upper surface of the medial edge of the parahippocampal gyrus in close proximity to the choroidal fissure. D, axial magnetic resonance imaging scan of a cadaver using the Stealth workstation demonstrating the segments of the perimesencephalic cisterns. E, coronal magnetic resonance imaging scan demonstrates the “C” shape of the ambient cistern. A., artery; AChA, anterior choroidal artery; Ant. Seg., anterior segment of uncus; Basal V., basal vein; Body, body of hippocampus; Chor. Plex., choroid plexus; Cist., cistern; CN III, oculomotor nerve; Head, head of hippocampus; Inf. Temp. A., inferior temporal artery; Lat. Gen. Body, lateral geniculate body; LPChA, lateral posterior choroidal artery; MPChA, medial posterior choroidal artery; Optic Tr., optic tract; Parahippo. Gyrus, parahippocampal gyrus; PCoA, posterior communicating artery; Post. Seg., posterior segment of uncus; SCA, superior cerebellar artery; Tail, tail of hippocampus. (Images courtesy of AL Rhoton, Jr.)

We have previously described the anatomy of the PCA and divided it into relevant segments (52). Briefly, the P1 segment begins at the basilar apex and ends at the insertion with the posterior communicating artery. The P2 segment is divided into the anterior portion (P2a) and the P2p. The P2a extends from the junction of the posterior communicating artery with the PCA and ends at the posterior margin of the crural cistern at the back edge of the cerebral peduncle. The P2p begins at the posterior border of the crural cistern and ends at the lateral edge of the midbrain colliculi. The P2p often courses superiorly and laterally within the ambient cistern to lie on the superior surface of the parahippocampal gyrus. The P3 segment traverses the quadrigeminal cistern, and the P4 segment is formed by the cortical branches arising from the PCA along its course (Fig. 1).

The choroidal arteries are intimately associated with the perimesencephalic cisterns. The anterior choroidal artery arises several millimeters distal to the takeoff of the posterior communicating artery from the posterior surface of the internal carotid artery and courses along the anterior and proximal posterior uncal segment before entering the lateral ventricle through the inferior choroidal point (3, 8, 9, 45). The medial posterior choroidal artery most commonly arises from the P1; however, a significant number arise more distally from the P2. The medial posterior choroidal artery travels inferior and medial to the PCA through the crural and ambient cisterns and turns medially to enter the quadrigeminal cistern (9, 23, 30, 45). The artery then turns forward to enter the velum interpositum and supplies the choroid plexus in the roof of the third ventricle (9, 30, 45). The lateral posterior choroidal arteries arise most commonly from the P2p as one or several branches, course laterally along the upper edge of the parahippocampal gyrus within the ambient cistern, and pass through the choroidal fissure to enter the posterior part of the temporal horn and atrium (Fig. 1, A and C) (8, 9, 24, 30).

The PCA gives rise to cortical branches along its course through the crural, ambient, and quadrigeminal cisterns. From proximal to distal, these branches include the inferior temporal, parieto-occipital, calcarine, and splenial arteries (52). The inferior temporal arteries comprise a group of vessels that include the hippocampal, anterior temporal, middle temporal, and posterior temporal branches (Fig. 1, A–C) (52).

RESULTS

Transsylvian Pretemporal Approach

The heads were placed in a standard surgical position for approaching the basilar artery through the transsylvian exposure (4, 15, 34, 35, 49). The sylvian fissure was widely dissected and opened along its entire length. The cisterns surrounding the optic nerve and internal carotid artery were likewise dissected. After exposure of the areas between the optic and oculomotor nerves and carotid arteries, attention was directed to the exposure of the perimesencephalic cisterns. The temporal lobe was retracted posterolaterally away from the frontal operculum to expose the basilar tip and origin of the PCA. At this point, head positioning and scope angulation were optimized to expose the course of the PCA around the cerebral peduncle. The posterior limit of temporal lobe retraction was at the posterior edge of Meckel’s cave (Fig. 2, A–D).

Figure 2. Pretemporal transsylvian approach. A, specimen showing right orbitozygomatic craniotomy exposing the sylvian fissure and frontal and temporal lobes. The inset shows the site of the twopiece orbitozygomatic craniotomy. B, specimen showing widely opened proximal portion of the sylvian fissure, exposing the M2 branches and insular surface. C, specimen showing opened opticocarotid triangle, located between the optic nerve, carotid artery, and A1 segment of the anterior cerebral artery, and opened carotid-oculomotor space, located between the oculomotor nerve and carotid artery. Note the limen insula obstructing the view into the crural cistern. D, specimen showing temporal lobe retracted posteriorly and proximal M1 elevated to expose the P2a in the anterior part of the crural cistern. The P1s, P2a, anterior choroidal artery, and PCA come into view. E–G, Stealth magnetic resonance imaging scans revealing the limits (arrows) of the exposure obtained with the pretemporal approach. Anterior exposure through the approach is excellent and provides access to the interpeduncular cistern. The posterior and superior exposures are limited by the limen insula. The posterior limit of exposure is at the posterior edge of the cerebral peduncle, and the upper limit is on the upper third of the uncus. A1, A1 segment of the anterior communicating artery; AChA, anterior choroidal artery; CN II, optic nerve; CN III, oculomotor nerve; ICA, internal carotid artery; M1, M1 segment of middle cerebral artery; M2, M2 segment of the middle cerebral artery; PCoA, posterior communicating artery. (Images courtesy of AL Rhoton, Jr.)

In the six hemispheres that were studied using the Stealth Image Guidance workstation, several measurements and localizations were made. In particular, the anterior, posterior, superior, and inferior limits of exposure within the perimesencephalic cistern were determined. The pretemporal approach exposed the interpeduncular and crural cisterns but not the ambient and quadrigeminal portions. The major anatomic limitations to exposure included the limen insula, the medially projecting uncal apex, and the excessive retraction of the posterior temporal lobe required to reach some areas (Fig. 2, E–G).

The approach provided excellent exposure anterior to the midbrain and included the internal carotid, anterior cerebral, and middle cerebral artery complexes. The PCA origin and the P1 and P2a were exposed in all cases. In addition, we were able to expose the origin and most of the cisternal segment of the anterior choroidal artery. The medial posterior choroidal artery origin was identified in 9 of 10 hemispheres arising from the P1 or P2a. In one hemisphere, the medial posterior choroidal artery arose from the P2p within the ambient cistern and could not be exposed. The lateral posterior choroidal vessels, which arise most commonly from the P2p, could not be exposed through the transsylvian approach. In addition, the P2p and P3 were inaccessible through this approach (Table 1).

Subtemporal Approach

Three variations of the subtemporal surgical approach were studied: the anterior, middle, and posterior extensions (5, 7, 10, 11, 20, 27, 35, 39, 43). The approach involves placing the sagittal suture parallel to the floor with the vertex angled inferiorly to allow for maximal visualization along the tentorial surface to the perimesencephalic cisterns. The zygomatic arch and squamosal and petrous portions of the temporal bones were drilled to form a flat trajectory along the floor of the middle fossa. The arachnoidal trabeculae connecting the mesial temporal lobe to the tentorial edge were sharply dissected and removed to expose the underlying structures (Fig. 3, A–C).

Figure 3. A–C, left subtemporal approach to the perimesencephalic cisterns. A, specimen showing anterior subtemporal approach exposing the interpeduncular and lower half of the crural and ambient cisterns. Structures exposed include the internal carotid, anterior choroidal, posterior communicating, basilar, superior cerebellar and medial posterior choroidal arteries, P1 and P2a, and anterior and posterior segments and apex of the uncus. B, specimen showing middle subtemporal approach exposing the P2a within the crural cistern, P2p within the ambient cistern, medial posterior choroidal and superior cerebellar arteries, trochlear nerve, and tentorial edge. C, specimen showing posterior subtemporal exposure with view into anterior quadrigeminal cistern exposing the collicular plate, trochlear nerve, and basal vein. D–G, Stealth magnetic resonance imaging scans demonstrating the limits (arrows) of the approach. D, left anterior subtemporal exposure provides access to the interpeduncular cistern and across to the medial aspect of the contralateral cerebral peduncle. E, posterior limit of exposure in hemispheres not limited by an anterior position of the vein of Labbé was the lateral part of the quadrigeminal cistern on the side ipsilateral to the approach. F, posterior limit of exposure ipsilateral to the approach in hemispheres in which an anterior position of the vein of Labbé limits temporal lobe retraction. G, superior limit of exposure within the ambient cistern was limited to the lower one-half to two-thirds of the cistern because of the prominence of the medial edge of the parahippocampal gyrus. Reaching the upper part of the cistern would require excessive temporal lobe retraction. AChA, anterior choroidal artery; Ambient, ambient cistern; Ant. Seg., anterior segment of uncus; Apex, apex of uncus; Bas. Tr., basilar trunk; Basal V., basal vein of Rosenthal; CN III, oculomotor nerve; CN IV, trochlear nerve; Crural, crural cistern; ICA, internal carotid artery; SCA, superior cerebellar artery; Tent edge, tentorial edge; MPChA, medial posterior choroidal artery; PCoA, posterior communicating artery; Post. Seg., posterior segment of the uncus. (Images courtesy of AL Rhoton, Jr.)

In all specimens, the interpeduncular, crural, and lower half of the ambient cistern could be exposed subtemporally (Fig. 3, D–G). In 6 of 10 specimens, the posterior subtemporal route could access the quadrigeminal cistern (Fig. 3E). In 4 specimens, the vein of Labbé blocked exposure of the posterior part of the ambient cistern (Fig. 3F). Another anatomic obstacle to exposure through this approach is the parahippocampal gyrus. The upper one-third to one-half of the ambient cistern, which extends above the rounded medial edge of the parahippocampal gyrus, was inaccessible from the subtemporal approach (Fig. 3G).

The basilar tip, P1, P2a, and medial posterior choroidal artery were exposed in all 10 specimens. The origin and most of the cisternal segment of the anterior choroidal artery could also be exposed through the subtemporal approach. The proximal and inferior surface of the P2p was accessible in three hemispheres. In seven hemispheres, the P2p coursed superiorly at the posterior edge of the cerebral peduncle and became hidden from view above the medial edge of the parahippocampal gyrus (Fig. 3, A and B). The lateral posterior choroidal arteries and P3 were inaccessible in all hemispheres (Table 1).

Occipital Transtentorial and Infratentorial Supracerebellar Approaches

Two posterior approaches, one directed above and one below the tentorium, were examined. The approaches examined were the occipital transtentorial and supracerebellar transtentorial approaches (1, 2, 18, 19, 28, 29, 31, 32, 35, 37, 40, 46, 47, 51, 53). Both were found to provide nearly identical exposure of the perimesencephalic cisterns and structures within. Each involves a paramedian incision through the tentorium. The obvious difference involves retraction of the cerebellum versus retraction of the occipital pole (Figs. 4 and 5).

Figure 4. Right occipital transtentorial approach. A, specimen showing posterior view, in which the inset demonstrates the route of the exposure. The tentorium was divided adjacent to the straight sinus. The exposure included the P2a, P2p, P3, pineal gland, colliculi, superior cerebellar artery, basal vein and vein of Galen, trochlear nerve, and inferior temporal branches of the PCA. B, specimen showing enlarged view above the cerebellum into the ambient cistern. The approach is directed lateral to the apex of the cerebellar vermis. The P2a has ascended to be hidden above the medial edge of the parahippocampal gyrus in the upper part of the ambient cistern. The P3 re-emerges in the quadrigeminal cistern. C–G, Stealth magnetic resonance imaging scans demonstrating the limits (arrows) of exposure. C, anterior limit of exposure is the back edge of the crural cistern. D and E, approach provides excellent exposure of the quadrigeminal cistern. F, working distance to the deepest point of exposure in the six hemispheres averaged 8 cm. G, trajectory of approach to the anterior limit of exposure. H, superior limit of exposure in ambient cistern. Basal V., basal vein; CN IV, trochlear nerve; Inf. Temp. A., inferior temporal artery; Parahippo. Gyrus, parahippocampal gyrus; Pineal, pineal gland; SCA, superior cerebellar artery; Sup. Col., superior colliculus; Tent., tentorium; V. of Galen, vein of Galen. (Images courtesy of AL Rhoton, Jr.)

Figure 5. Infratentorial supracerebellar transtentorial approach. A, specimen showing posterior view of the cerebellum. Note the bridging veins blocking access to the quadrigeminal cistern. B, specimen showing infratentorial midline view of the quadrigeminal cistern. Exposed structures include the pineal gland, complex of veins emptying into the vein of Galen, P3, superior cerebellar artery, and medial posterior choroidal artery. C, specimen showing left paramedian approach with the tentorium intact exposing the collicular plate, fourth nerve, and superior cerebellar artery. The P3 is exposed above and medial to the tentorial edge. D, specimen showing view with the tentorium divided exposing the P2p in the ambient cistern and the P2a in the posterior crural cistern. The distal P2p ascends in the anterior part of the ambient cistern to become hidden above the medial edge of the parahippocampal gyrus. E–J, Stealth magnetic resonance imaging scans demonstrating the limits (arrows) of exposure. E, anterior limit of exposure is at the back edge of the crural cistern. F and G, approach provides excellent exposure of the quadrigeminal cistern. H, trajectory of approach to the anterior middle portion of the ambient cistern. I, working distance from the posterior cortical surface to the most anterior point of exposure averaged 7 to 8 cm. J, superior limit of the exposure extended into the upper one-third of the ambient cistern, similar to that obtained through the occipital transtentorial approach. Basal V., basal vein of Rosenthal; Cist., cistern; CN IV, trochlear nerve; Colliculi, superior and inferior colliculi; Inf. Temporal A., inferior temporal artery; Inf. Vermian Vein, inferior vermian vein; Int. Cerebral V., internal cerebral vein; MPChA, medial posterior choroidal artery; Parahippo. Gyr., parahippocampal gyrus; PICA, posterior inferior cerebellar artery; Pineal, pineal gland; Sup. Colliculus, superior colliculus; Tent. Edge, tentorium; Transv. Sinus, transverse sinus; V. of Galen, vein of Galen. (Images courtesy of AL Rhoton, Jr.)

Both the occipital transtentorial approach and the supracerebellar transtentorial approach allow for excellent exposure of the quadrigeminal and ambient cisterns as well as access to lesions extending below the tentorial edge (Figs. 4 and 5). The posterior portion of the crural cistern can be reached through both approaches, with a slight increase in exposure gained through the supracerebellar route (Figs. 4C and 5E). Compared with the more anterior approaches, the working distance is significantly greater when accessing the anterior ambient cistern and posterior crural cistern (Figs. 4F and 5I). The average working distance from the surface of the cortex to the anterior ambient cistern for the posterior approaches was approximately 8 cm, whereas the working distance for the anterior approaches ranged between 4 and 5 cm.

These posterior approaches allow for excellent exposure of the P3. The P1 is inaccessible through these approaches, and only the posterior third of the P2a can be exposed. In four hemispheres, the inferior surface of the P2p could be exposed. In two hemispheres, the P2p existed as two trunks, a superior trunk and an inferior trunk. In these hemispheres, the inferior trunk could be accessed, whereas the superior trunk was hidden from view above the medial edge of the parahippocampal gyrus. In the remaining four hemispheres, the P2p was inaccessible above the medial edge of the parahippocampal gyrus (Figs. 4B and 5D; Table 1).

Both approaches allowed similar access to the choroidal vessels. The anterior choroidal artery could not be accessed in any of the hemispheres because it entered the temporal horn anterior to the exposure in the posterior crural cistern. The ambient and quadrigeminal portions of the medial posterior choroidal artery could be accessed but not the origin from the P1. The lateral posterior choroidal arteries were inaccessible secondary to their origin from the superior or lateral surface of the P2p segment and subsequent lateral course toward the temporal horn above the parahippocampal gyrus (Table 1).

Transtemporal Transchoroidal and Transinsular Transchoroidal Approaches

The perimesencephalic cisterns can be accessed through the choroidal fissure of the temporal horn (16, 24, 25, 26, 30, 45). The choroidal fissure is the cleft between the pulvinar of the thalamus and the fimbria of the fornix along which the choroid plexus in the temporal horn is attached. The fissure begins at the inferior choroidal point, where the anterior choroidal artery enters the temporal horn at the posterior aspect of the crural cistern and posterior edge of the posterior uncal segment, marking the beginning of the plexal portion of the artery. In addition to the anterior choroidal artery, the lateral posterior choroidal artery branches enter the ventricular system through the fissure within the posterior part of the temporal horn and atrium (24, 30, 45). Two approaches through the choroidal fissure were examined, one through the inferior temporal lobe and the other through the inferior limiting sulcus of the insula (Figs. 6–9) (12, 16, 25, 26, 36, 42, 45, 50).

Figure 6. Transtemporal transchoroidal approach. A, specimen showing right temporal craniotomy exposing the inferior temporal gyrus. The inset shows the site of the scalp incision and craniotomy. B, specimen showing temporal horn of the lateral ventricle entered through a 2-cm corticotomy in the inferior temporal gyrus to expose the head of the hippocampus, choroid plexus, and choroidal fissure. C, specimen showing choroidal fissure opened on the forniceal side by separating the choroid plexus from the fimbria of the fornix to preserve the draining veins and perforating arteries, which traverse the thalamic side of the fissure. Note the lateral geniculate body, anterior and lateral posterior choroidal arteries, and basal vein. D, specimen showing uncal recess positioned in the anterior tip of the temporal horn between the head of the hippocampus and the amygdala. The anterior choroidal artery enters the temporal horn at the inferior choroidal point located just behind the hippocampal head and supplies the choroid plexus. E, specimen showing enlarged view of the P2p, lateral posterior choroidal artery, and basal vein coursing in the ambient cistern. F, specimen showing quadrigeminal cistern and P3 accessed through the posterior portion of the approach directed through the choroidal fissure. G, specimen showing retraction of the hippocampus exposing the inferior temporal artery originating from the P2p. H, specimen showing transamygdala extension of the transchoroidal approach. An incision has been carried forward through the amygdala to expose the structures within the crural and interpeduncular cisterns, including the P2a; amygdala; posterior communicating, internal carotid, anterior choroidal, and medial posterior choroidal arteries; and optic and oculomotor nerves. AChA, anterior choroidal artery; Basal V., basal vein; Cer. Ped., cerebral peduncle; Chor. Fiss., choroidal fissure; Chor. Plex., choroid plexus; CN II, optic nerve; CN III, oculomotor nerve; Hip. A., hippocampal artery; ICA, internal carotid artery; Inf. Temp. A., inferior temporal artery; Inf. Temp. Gyrus, inferior temporal gyrus; Lat. Gen. Body, lateral geniculate body; LPChA, lateral posterior choroidal artery; MPChA, medial posterior choroidal artery; Parahippo. Gyrus, parahippocampal gyrus; PCoA, posterior communicating artery; Pulvinar, pulvinar of thalamus; Quadrigeminal, quadrigeminal cistern; Sphenopar. Sinus, sphenoparietal sinus. (Images courtesy of AL Rhoton, Jr.)

Figure 7. Stealth magnetic resonance imaging scans demonstrating the exposure gained through the transtemporal-transchoroidal approach. Arrows, the limits of the exposure. A, trajectory through the inferior temporal gyrus and temporal horn to the ambient cistern. B, distance from the lateral cortex to the temporal horn averaged 2 to 2.5 cm. C, trajectory to the posterior ambient cistern from the posterior margin of the corticotomy through the inferior temporal gyrus. The distance to the ambient cistern from the cortical surface was approximately 4.0 to 4.5 cm. D, right-sided anterior limit of exposure at the inferior choroidal point. E, left-sided posterior limit of exposure in the proximal quadrigeminal cistern. F, inferior limit of exposure within the ambient cistern. Note the inability to access the lower ambient cistern. G, upper limit of exposure at the choroidal fissure. (Images courtesy of AL Rhoton, Jr.)

Figure 8. Transinsular transchoroidal approach to the perimesencephalic cisterns. A, specimen showing right orbitozygomatic craniotomy with exposure of the frontal and temporal lobes along the sylvian fissure. The inset shows the site of the scalp incision and craniotomy. B, the sylvian fissure is widely opened to expose the insular cortex and insular branches of the M2 segment of the middle cerebral artery. C, specimen showing enlarged view. The inferior limiting sulcus of the insula has been exposed. D, specimen showing cortical incision through the anterior portion of the inferior limiting sulcus parallel to M2 branches opening into the temporal horn and exposing the choroid plexus and head of hippocampus. The temporal horn usually lies approximately 0.5 cm deep to the inferior limiting sulcus. E, specimen showing choroidal fissure opened by dividing the attachment of the choroid plexus to the fimbria of the fornix and retracting the choroid plexus toward the thalamus to expose the lateral geniculate body, basal vein, and P2p. F, specimen showing P2p elevated to expose the depths of the ambient cistern and an inferior temporal branch of the P2p passing to the lower surface of the temporal lobe. Basal V., basal vein; Cent. Insular Sulcus, central insular sulcus; Chor. Plexus, choroid plexus; Fimbria, fimbria of fornix; Hippo., hippocampus; Inf. Limiting Sulcus, inferior limiting sulcus of insula; Inf. Temp. A., inferior temporal artery; Lat. Gen. Body, lateral geniculate body; LPChA, lateral posterior choroidal artery; M2, M2 segment of middle cerebral artery; Sylvian Fiss., sylvian fissure; Sylvian V., sylvian vein. (Images courtesy of AL Rhoton, Jr.)

Figure 9. Stealth magnetic resonance imaging scans of the transinsular transchoroidal approach. A and B, trajectory of the approach through the sylvian fissure and inferior limiting sulcus to the ambient cistern. C, superior limit of the exposure through the choroidal fissure to the ambient cistern. D, anterior limit of the exposure at the inferior choroidal point in the posterior end of the crural cistern. E, posterior limit of exposure, in most hemispheres, at the posterior ambient cistern. F, inferior limit of exposure within the lower part of the ambient cistern. Note how the superior-to-inferior trajectory of the transinsular approach allows for greater inferior exposure of the ambient cistern than through the transtemporal transchoroidal approach as illustrated in Figure 7. Arrows, the limits of the exposure. (Images courtesy of AL Rhoton, Jr.)

In the 10 hemispheres studied, the temporal horn was found to be located 2.5 to 3.5 cm posterior to the anterior tip of the temporal lobe and approximately 2 to 2.5 cm deep to the surface of the middle temporal gyrus (Fig. 7B). The transtemporal transchoroidal approach involves making a 2-cm corticectomy in the inferior temporal gyrus and opening the lateral wall of the temporal horn (Fig. 6, A and B). After entering the temporal horn, the choroidal fissure is opened between the fimbria and choroid plexus to avoid the thalamic draining veins and perforating arteries traversing the thalamic side of the fissure (Fig. 6C). At this point, the upper portion of the posterior crural cistern and the ambient cistern are exposed. In most hemispheres, the lateral geniculate body in the roof of the cistern and the basal vein are the first structures encountered after the initial opening of the fissure (Fig. 6C). With minimal retraction of the hippocampus inferiorly, the upper half of the ambient cistern and related segments of the PCA can be exposed (Fig. 6D). More extensive retraction and dissection provide more inferior exposure of the ambient cistern as well as exposure of the posterior crural and anterior quadrigeminal cisterns (Fig. 6, E–G). The major drawback to the transtemporal transchoroidal approach is the need to perform a corticectomy in the temporal lobe. Several authors have reported success in using the approach for lesions in the ambient cistern with minimal morbidity (16, 25, 26, 36). Another limitation of the approach is the vein of Labbé. In 4 of 10 hemispheres, the vein of Labbé extended far enough anteriorly to limit the cortical incision in the temporal lobe.

Another route to access the choroidal fissure of the temporal horn is through the inferior limiting sulcus of the insula (50). The approach involves making a 2-cm incision in the inferior limiting sulcus to gain access to the temporal horn. The incision is usually made between the insular branches of the middle cerebral artery (Fig. 8D). The depth of the temporal horn is approximately 5 mm from the insular surface. Once the temporal horn is entered, the choroidal fissure is opened in an identical manner to that described for the transtemporal transchoroidal approach (Fig. 8, A–F). A variation of the transinsular transchoroidal approach has been extensively used and described in performing amygdalohippocampectomies for temporal lobe epilepsy as well as for approaching lesions in the ambient cistern (50). Given the extensive use of this approach in clinical practice, one would expect minimal morbidity secondary to the incision along the anterior portion of the inferior limiting sulcus (12, 42, 50).

The transchoroidal approaches provide excellent exposure of the lateral posterior choroidal artery, which enters the temporal horn through the choroidal fissure. In addition, the P2p was exposed transchoroidally in 9 of 10 hemispheres examined. In one hemisphere, the P2p was located on the undersurface of the parahippocampal gyrus and could not be exposed transchoroidally. Both transchoroidal approaches exposed the upper half of the ambient cistern, the posterior third of the crural cistern, and the adjacent part of the quadrigeminal cistern (Figs. 7 and 9). The transinsular variation, given the superior-to-inferior trajectory of the approach, allowed for greater exposure of the inferior portion of the ambient cistern (Figs. 7F and 9F). In addition, we were able to gain access to the posterior P2a and anterior P3 through both variations. The plexal portion of the anterior choroidal artery was exposed within the temporal horn (Table 1).

Limitations of the approaches through the choroidal fissure include an inability to access the medial posterior choroidal artery, the origin and cisternal segments of the anterior choroidal artery, the P1, and most of the P2a and P3. The medial posterior choroidal artery lies deep within the ambient cistern below the PCA and is intermingled with multiple brainstem perforators, making identification through these approaches difficult (Table 1).

DISCUSSION

Lesions in and around the perimesencephalic cisterns present unique challenges to the neurosurgeon. Success requires a thorough knowledge of the anatomy and an understanding of the limitations of the approaches used to access the region. The deep location, narrow confines, and density of critically important vascular structures add to the challenges. In the present study, we have examined six approaches to the region in an attempt to define the surgical anatomy of the region and describe the limitations of the various approaches (Fig. 10).

Figure 10. Composite diagram demonstrating the exposure gained through six approaches to the perimesencephalic cisterns. Axial (left) and coronal (right) images are shown. A, left image demonstrates the axial exposure gained through the pretemporal (pink) and occipital transtentorial and supracerebellar transtentorial (blue) approaches. The right image demonstrates the coronal exposure of the ambient cistern provided by the occipital transtentorial and supracerebellar transtentorial approaches. The pretemporal approach exposes the interpeduncular and crural cisterns but not the ambient or quadrigeminal cisterns (left, pink). The occipital transtentorial and supracerebellar transtentorial approaches expose the quadrigeminal and ambient cisterns but not the crural or interpeduncular cisterns (left, blue). The occipital transtentorial and supracerebellar transtentorial approaches provide access to the lower two-thirds of the ambient cistern (right image), whereas the upper one-third of the ambient cistern is obscured by the medial edge of the parahippocampal gyrus. B, left image demonstrates the axial exposure of the perimesencephalic cisterns through the subtemporal approach. The subtemporal approach consistently exposes the interpeduncular, crural, and ambient cisterns (orange solid). Exposure of the posterior ambient and quadrigeminal cisterns is more variable and depends on the position of the vein of Labbé (orange hatched). The right image, in the coronal plane, demonstrates the extent of exposure of the ambient cistern through the subtemporal approach. The lower one-half of the ambient cistern was exposed subtemporally in all specimens (orange solid). The exposure of the upper one-half of the ambient cistern was blocked in the subtemporal approach by the medial edge of the parahippocampal gyrus and the position of the vein of Labbé. Excessive retraction would be required to expose the upper part of the ambient cistern. C, left image, transinsular and transtemporal transchoroidal approaches provided access to the ambient cistern in all hemispheres (green solid). Exposure of the quadrigeminal cistern was more variable and was limited by the position of the tail of the hippocampus (green hatched). In the right image, both the transinsular (upper green arrow) and transtemporal transchoroidal (lower green arrow) approaches exposed the upper one-half to two-thirds of the ambient cistern (solid green). The transinsular transchoroidal approach accessed the lower ambient cistern (green hatched). The added inferior exposure gained through the transinsular approach is a result of the superior-to-inferior trajectory through the inferior limiting sulcus of the insula (right image, upper green arrow). (Images courtesy of AL Rhoton, Jr.)

The PCA is the dominant vascular structure within the perimesencephalic cisterns. The segments of the PCA are named according to the cistern in which they reside, and each segment requires a unique surgical approach (6, 16, 25, 36, 44, 52). Basilar apex, P1-posterior communicating artery, and posterior communicating artery-P2a aneurysms can be accessed through the transsylvian or anterior subtemporal approach (4–7, 11, 15, 20, 33–35, 38, 49). The occipital transtentorial and infratentorial supracerebellar approaches have been considered the best approaches for aneurysms of the P3 segment (36, 44).

Aneurysms of the P2, particularly the P2p, have historically been the most challenging lesions to approach (16, 25, 36, 44). The P2p runs within the ambient cistern in close association with the parahippocampal gyrus. The P2 typically ascends within the ambient cistern and usually lies on the superior surface of the parahippocampal gyrus. The ambient cistern is “C” shaped in coronal cross section because it extends around the rounded medial edge of the parahippocampal gyrus. When the P2p lies within the upper portion of the “C,” the gyrus limits the exposure of this segment when approached from below through the subtemporal, occipital transtentorial, and infratentorial supracerebellar approaches. Only the proximal and inferior surface of the P2p was accessible through the subtemporal route in 3 of 10 hemispheres. In the other 7 hemispheres, a portion of the parahippocampal gyrus would have to be resected to expose the P2p subtemporally (14). The P2p was successfully exposed in 4 of 10 hemispheres through the occipital transtentorial and infratentorial supracerebellar approaches. In 2 hemispheres, the P2p divided into an inferior trunk, which could be exposed below the parahippocampal gyrus, and a superior trunk, which was hidden above the parahippocampal gyrus. In the remaining 4 hemispheres, the P2p was inaccessible by the occipital transtentorial and infratentorial supracerebellar approaches because it coursed on the upper surface of the parahippocampal gyrus.

The transchoroidal approach provides access to the upper ambient cistern. In 9 of 10 hemispheres, the P2p was exposed transchoroidally. In 1 hemisphere, the P2p was positioned in the lower half of the ambient cistern and could not be exposed transchoroidally, because the upper part of the parahippocampal gyrus blocked access to the P2p. Some authors have suggested that the location of the P2 segment aneurysm in relation to the inferior choroidal point, as revealed on angiography, is a critical landmark for selecting the appropriate surgical approach to these lesions (16, 44). Ikeda et al. (16) recommend using the transchoroidal route for distal P2 aneurysms when the PCA is at or above the level of the inferior choroidal point . On the basis of our current study, we think that the midpoint of the rounded medial edge of the parahippocampal gyrus provides an equally important landmark. Moving superiorly from the midpoint of the medial edge improves the exposure through the transchoroidal approach but worsens the exposure through the subtemporal, occipital transtentorial, and supracerebellar approaches. Conversely, moving inferiorly from the midpoint of the gyrus improves the exposures gained through the subtemporal, occipital, and supracerebellar approaches but worsens the exposure of the transchoroidal approaches (Fig. 10).

The anterior, medial posterior, and lateral posterior choroidal arteries are intimately associated with the perimesencephalic cisterns (8, 24, 45). Specific surgical approaches are required to expose these arteries. The anterior choroidal artery arises from the internal carotid artery just distal to the PCA and traverses the anterior two-thirds of the crural cistern before entering the temporal horn at the inferior choroidal point. The origin and cisternal segments can only be approached through the transsylvian pretemporal and anterior subtemporal routes. The plexal portion is only accessible transchoroidally. The medial posterior choroidal artery has the longest course of the three choroidal arteries, and no single approach can expose all the segments of the vessel. The artery typically arises as a single trunk from the P1 and traverses the crural, ambient, and quadrigeminal cisterns medial to and below the PCA before entering the posterior roof of the third ventricle. The transsylvian pretemporal approach can expose the origin of the vessel and the crural segment. In 1 of 10 hemispheres studied, the origin of the vessel was from the P2p and was not accessible through the transsylvian route. The subtemporal approach exposed the origin as well as the crural, ambient, and proximal quadrigeminal segments of the medial posterior choroidal artery in all 10 hemispheres studied. Access to the distal quadrigeminal portion of the artery was limited because of the need for excessive temporal lobe retraction and/or the position of the vein of Labbé. Exposure of the medial posterior choroidal artery through the two transchoroidal approaches was difficult because of the fact that the artery has a relatively low-lying position within the cistern and visualization is obstructed by the overlying PCA and multiple brainstem perforators. The lateral posterior choroidal artery proved to be the most difficult artery to identify and expose. The artery typically arises as one or several branches off the distal P2 and courses directly lateral on the superior surface of the parahippocampal gyrus to enter the temporal horn. Only the transchoroidal approaches could reliably expose the artery.

When approaching lesions involving the P3 or lesions extending for any considerable distance into the quadrigeminal cistern, the occipital transtentorial or infratentorial supracerebellar approach should be selected. None of the other approaches reliably exposed these structures.

All the approaches examined have limitations in their ability to expose critical structures within the perimesencephalic cisterns. These limitations may not be readily apparent on preoperative imaging studies but may become apparent during the course of an operation. Several of these approaches can be combined using the same craniotomy to extend the operative field. For example, combing the transsylvian pretemporal approach with the transinsular transchoroidal approach allows the surgeon to gain access to the ambient cistern. In addition, through this combined transsylvian approach, the surgeon would have access to the origin, cisternal, and plexal portions of the anterior choroidal artery and lateral posterior choroidal artery as well as access to the P1, P2a, and P2p. The combined exposure could be critical when dealing with a complex arteriovenous malformation or tumors receiving blood supply from multiple choroidal arteries (13, 14, 17, 21, 41, 42, 48).

Another combined approach providing extended exposure and flexibility during surgery is the combination of the subtemporal and transtemporal transchoroidal approaches. The subtemporal approach is limited in its ability to expose the upper ambient cistern, the P2p, the lateral posterior choroidal artery, and the plexal portion of the anterior choroidal artery. However, the approach provides excellent exposure of the interpeduncular cistern, P1, P2a, lower ambient cistern, medial posterior choroidal artery, and origin of the anterior choroidal artery. Having the ability to add the transchoroidal exposure by opening through the lower part of the temporal lobe and temporal horn during the course of a subtemporal approach could greatly facilitate the ability to deal with complex arteriovenous malformations, tumors, and P2p aneurysms.

The transtemporal transchoroidal approach can be extended during the course of an operation to provide access to the anterior midbrain. Making an incision through the amygdala, as would be done for a temporal lobectomy, provides access to the internal carotid artery, P1, P2a, PCA, anterior choroidal artery, and medial posterior choroidal artery as well as the crural and interpeduncular cisterns (Fig. 6H). The extension through the amygdala can be used to deal with unexpected difficulties in exposing pathological findings or to provide proximal control when dealing with aneurysms.

When planning surgical approaches to the perimesencephalic cisterns, the surgeon must anticipate the critical structures needed in the exposure. With this understanding, the appropriate approach or combination of approaches can be selected. The transchoroidal extensions of the transsylvian pretemporal and subtemporal approaches can be added during the course of surgery to extend the exposure of these more standard approaches.

Impact and Importance

Approaching lesions within the perimesencephalic cisterns presents unique challenges to the neurosurgeon. The anatomy of this region is complex, with numerous arteries, perforating vessels, deep venous drainage pathways, and cranial nerves coursing through its confines. The perimesencephalic cisterns lie deep within the brain, and portions are shielded by overlying structures, including the temporal lobe, parahippocampal gyrus, limen insula, tentorial incisura, and vein of Labbé. No single surgical approach can provide access to the entire perimesencephalic cistern, but approaches must be tailored to a particular patient’s pathological findings with a thorough understanding of the anatomy of this region. We present the microsurgical anatomy of six approaches to the region, paying particular attention to the exposure of the PCA and its major branches. The approaches are compared using a novel application of image guidance.

Content from: Ulm AJ, Tanriover N, Kawashima M, Campero A, Bova FJ, Rhoton AL Jr. Microsurgical approaches to the perimesencephalic cisterns and related segments of the posterior cerebral artery: comparison using a novel application of image guidance. Neurosurgery 2004;54:1313–1327, 10.1227/01.NEU.0000126129.68707.E7. With permission of Oxford University Press on behalf of the Congress of Neurological Surgeons.

The Neurosurgical Atlas is honored to maintain the legacy of Albert L. Rhoton, Jr., MD.

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